Interpreting the Response to Loading Changes in a Mixed-Culture Completely Stirred Tank Reactor

Mathematical modeling and carefully controlled laboratory experiments are used to quantify the relationships among nitrifying and heterotrophic bacteria and the key constituents of effluent quality. Mathematical model results are used to interpret experimentally measured traditional parameters, such as soluble chemical oxygen demand (COD) and total biomass. Although total biomass declines with a decrease in influent COD loading, soluble effluent quality remains nearly the same because the effluent soluble COD is controlled by biomass-associated products. Increases in ammonia-nitrogen (NH⁺₄-N) loading result in increases in nitrifier biomass, and the active heterotrophs also increase as a result of formation of soluble microbial products (SMP) by autotrophic nitrifiers. However, increased NH⁺₄-N loading causes a steady increase in effluent soluble COD, indicating that the SMP production by nitrifiers is only partially used by the heterotrophs. During the nonsteady-state period after a loading change, nitrification responds more slowly than heterotrophic processes and controls the time needed to reach the new steady state.